Method for shaping the edges of slices of semiconductor material

ABSTRACT

The invention concerns a method in which the edge of the slices is made to slide in contact with the surfaces of a pad soaked in acid substances and in which, during the process, the areas of the edge of the slices that have been in contact with the pad are periodically washed with a special liquid.

FIELD OF THE INVENTION

This invention concerns a method for shaping the edges of slices ofsemiconductor material, especially epitaxial material, and to anapparatus for carrying out the method.

BACKGROUND OF THE INVENTION

In the technology of producing wafers or slices of semiconductormaterial, a recurring problem is that of shaping the edge of the slicesin order to remove the circular protuberances ("epitaxial crowns") whichform on the edge of the slices as a result of the growth on them ofepitaxial layers and which, unless they are removed, can increasefragility of the slices and pose difficulties in the subsequentdiffusion and photomasking operations.

In order to shape the edge and remove the epitaxial crown, currentlyknown equipment carry out mechanical abrasion of the edge by means ofdiamond wheels or discs of abrasive paper.

This type of equipment, however, has the drawback of producing dust whenin operation, and dust notoriously represents a source of contaminationfor semiconductor layers.

OBJECT OF THE INVENTION

It is the object of this invention to provide a method of shaping theedges of substrates of semiconductor material which does not present theaforesaid problem.

SUMMARY OF THE INVENTION

For this purpose, in the method according to the invention the edge ofthe slice is made to slide in contact with the surfaces of a pad soakedin an acid substance and, during the process, the areas that have beenin contact with the pad are periodically washed with a liquid.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features of the invention will be more readily apparentfrom the following description and the accompanying drawing in which,

FIGS. 1a, 1b and 1c are schematic cross-sectional views of the edge of aslice, respectively, in the virgin state, after epitaxial growth, andafter the shaping process according to the invention;

FIG. 2a is a fragmentary cross-sectional view of a pad used in theprocess according to the invention;

FIG. 2b is an end view of the pad of FIG. 2a;

FIG. 3 is a plan view taken of an apparatus for carrying out the processaccording to the invention;

FIG. 4 is a cross-sectional view along the line A--A of FIG. 3; and

FIG. 5 is an enlarged view of a detail of FIG. 3.

SPECIFIC DESCRIPTION

FIGS. 1a, 1b and 1c show a schematical cross-sectional view of the edgeof the same slice, respectively, before the growth of an epitaxial layer(FIG. 1a), after growth (FIG. 1b) and, lastly, (FIG. 1c) at the end ofthe edge shaping process according to the invention. In particular, FIG.1b shows the epitaxial crown K which forms along the edge during thegrowth of the epitaxial layer I, said crown being considerably higher(in the region of 40%) than the layer itself.

FIGS. 2a and 2b show a pad used in the process according to theinvention. The pad consists of a cylinder-shaped body B made of flexiblematerial resistant to the acids used in the process and capable ofsoaking up the acids.

The pad is provided on one side with a series of notches C, thecross-sections of which, according to a plane passing through the axisof the pad, are V-shaped. On the bottom of each notch is the mouth of achannel D which passes radially through the pad until it communicateswith the internal cavity J of the cylinder F, which serves the dualpurpose of supporting the pad and of conveying the acid substances.

The purpose of the channels is to enable the mixture of acids present inthe cavity of the cylinder F (consisting, for example, of 10%hydrofluoric acid, 30% nitric acid and 60% acetic acid, by volume) tooverflow into the channels themselves and then soak uniformly throughthe entire body of the pad.

In the apparatus of FIGS. 3 and 4, the various parts represent:

B: a pad of the type shown in FIGS. 2a and 2b;

G and H: pairs of rollers;

L: a slice of semiconductor material resting on the rollers G and H andslidingly engaged by one part of its edge in a notch in the pad B;

M: a tank containing on the bottom continuously replaced deionizedwater;

N: a motor for driving the rollers G and H by means of a belt and pulleytransmission system O; and

Q and R: bars for supporting a slice-holding basket P (shown in FIG. 5).

FIG. 5 shows an enlarged view of the tank of FIG. 4, in which referenceP indicates a basket-shaped container designed to receive the slices andto hold them in a vertical position and equally spaced apart from oneanother by means of slots on the right hand side and on the bottom ofthe basket. Said container is also provided with apertures incorrespondence with the rollers G and H and with the pad B, which enablethe edge of each slice to engage with the surface of the aforesaidcylinders and with a respective notch in the pad.

During the process, the slices in the container are caused to rotatearound their own axes by means of the rollers G and H. As a result ofthe rotatory movement the edge of each slice brushes against the pad forone or more turns, thereby undergoing chemical attack by the acids inwhich the pad is soaked, with consequent removal of the surface incontact. The removal is lesser or greater in relation to the compositionof the acid and to the number of turns made by the slices.

During the course of the rotatory movement, the area of the edge of theslice that has been in contact with the pad runs through the water bothon the bottom of the tank (which is continuously replaced by means ofinlet and outlet apertures, not visible in the drawings), thus quenchingany acid which may still remain on the surface of the slice.

It is obvious that numerous modifications, adjustments or replacement ofelements with other functionally equivalent elements may be made to thepreviously described embodiments, given by way of illustration, withouthowever deviating from the scope of the following claims.

In particular, it is obvious that the shape of the notches in the padshown in FIG. 2 (which is symmetrical with respect to the planesperpendicular to the axis of the pad and crossing trough the axis ofeach channel) is not binding. In fact, said notches may be differentlyshaped and, in particular, asymmetrical whenever it is desired to obtainan equally asymmetrical shape on the edge of the slices.

We claim:
 1. A method of shaping an edge of a slice of a semiconductormaterial, comprising the steps of:(a) sliding an edge of a slice of asemiconductor material in cyclic contact with a surface of a pad soakedwith an acid substance capable of dissolving material from said slice;(b) periodically washing portions of the edge of said slice withdeionized water; and (c) contacting said portions after washing with thedeionized water again with said pad, said deionized water being providedin a bath below said pad, said slice is round, and said edge is slid incontact with said surface of said pad by rotating said slice to displacesaid edge through a notch formed in said pad and then through said bath.2. The method defined in claim 1, further comprising the step ofcontinuously replacing said bath.
 3. The method defined in claim 2wherein said slice is rotated in a vertical plane.
 4. The method definedin claim 3 wherein a plurality of said slices are rotated simultaneouslythrough respective notches in said pad and then into said bath.
 5. Themethod defined in claim 4 wherein said slices are rotated by supportingthem on a pair of parallel rollers and rotating said rollers.